Manufacturers of semiconductor chips and assemblies use automatic test equipment (“ATE”) to verify the performance of devices before the devices are shipped to customers. ATE systems typically include a “test head” and a “tester body.” The test head houses portions of the test system that are preferably located as close as possible to the device under test, and connects to the tester body via one or more cables. For testing electronic devices, the test head connects or “docks” with a peripheral. The peripheral feeds a series of devices to the ATE system for testing, and the ATE system tests the devices.
Constraints affecting semiconductor test processes often make it impractical to move the peripheral to the test head. In most manufacturing facilities, therefore, the peripheral that feeds the chips remains stationary, and the test head is moved into position for docking with the peripheral.
A device called a “manipulator” moves the test head to the peripheral. FIG. 1 shows an example of a manipulator 100 for supporting a test head 110. A manipulator of this type is disclosed in U.S. patent application Ser. No. 09/615,292, entitled “AUTOMATIC TEST MANIPULATOR WITH SUPPORT INTERNAL TO TEST HEAD,” which is hereby incorporated by reference. This manipulator is expected to be used with the Tiger™ test system, which is currently being developed by Teradyne, Inc., of Boston, Mass.
As shown in FIG. 1, a manipulator 100 supports a test head 110 from a region internal to the test head 110. The manipulator 100 rotates the test head upon a twist gear 114, and swings the test head at the end of a horizontal member 116 upon a swing bearing 122. The manipulator 100 raises and lowers the test head on linear bearings 124. A base 120 supports the manipulator 100, and preferably includes outriggers 126 to prevent the manipulator from tipping.
The manipulator 100 preferably includes actuators such as motors (not shown) on the twist gear 114, linear bearings 124, and swing bearing 122. The actuators move the test head to the peripheral, and orient the test head for docking. The test head is then docked with the peripheral by finely adjusting the position and orientation of the test head.
Manipulators commonly provide a range of “compliance” that allows a test head to be rotated about one or more axes as the test head and peripheral are being docked. The range is “compliant” because the test head literally complies with forces applied to the test head, which during docking tend to cause the mating surface of the test head to become coplanar with the mating surface of the peripheral. Without compliance, the manipulator would have to finely adjust the test head to a coplanar orientation with respect to the peripheral by precisely controlling the manipulator's actuators. For an example of a manipulator that automatically controls its actuators to achieve precise docking, see U.S. Pat. No. 5,949,002, entitled “Manipulator for Automatic Test Equipment with Active Compliance.”
Near the peripheral, the test head must be moved with great care. Both the test head and the peripheral include fragile electronic assemblies that can be damaged by collisions between the test head and the peripheral. Generally, the test head includes alignment pins for entering alignment bushings within the peripheral. During docking, the alignment pins must be made to enter the alignment bushings without bending or breaking them.
FIG. 2 is an exploded view of the test head 110 of FIG. 1, which illustrates how compliant movement of the test head 110 used in the Tiger™ test system can be achieved. As shown in FIG. 2, the test head 110 is composed of two portions 110a and 110b, which fasten to either side of a C-shaped stiffener 112. A central blade 210 extends from the manipulator 100 into the C-shaped stiffener 112, where it is held in place by a spherical bearing 228. The spherical bearing 228 mechanically couples the central blade 210 to the test head 110 via a transition insert 224, and allows the test head 110 to be rotated with respect to the central blade 210. The transition insert 224 has a fixed position relative to the test head (although it can be moved back and forth on rails to center the test head by rotating the lead screw 226). The transition insert 224 has a clearance area that completely surrounds the central blade 210. The clearance area allows the test head 110 to be rotated over a limited angular range about the spherical bearing 228. More specifically, the test head 110 can be rotated over approximately 5 degrees in each of the conventional directions of theta, tumble, and twist. These directions are illustrated, respectively, with the arrows 230, 232, and 236.
It is generally desirable that a test head be oriented toward the center of its compliance range about each axis for which compliance is provided, during the time when the test head is moved into position for docking. Centering each axis of a test head within its compliance range ensures that the test head always has some range of rotation available for providing compliant docking.
In the past, manipulators have used springs to bias each axis toward the center of its compliance range. FIG. 3 illustrates this approach for spring-biasing a conventional fork-arm manipulator. Fork arms 316a and 316b of a manipulator support a test head 310. Within each of the fork arms, springs 320a and 320b bias a shaft 318 that holds the test head toward the center of a compliance range along the length of the fork arms.
When the manipulator roughly aligns the test head 310 and presses the test head against the peripheral, the springs 320a and 320b adjust (one compresses, the other expands) in compliance with the applied force to allow the mating surfaces of the test head and peripheral to come together. When the test head is undocked from the peripheral, the springs 320a and 320b restore the orientation of the test head 310 to the center of the compliance range.
In the Tiger™ test system, the need to provide compliant docking of the test head has given rise to new challenges. The Tiger™ test head is extremely heavy, weighing approximately 1,270 kilograms (2,800 lbs.). We have recognized that a slight misalignment of the center of gravity of the test head from the location of the spherical bearing induces a large turning moment of the test head. In addition, cables attached to the test head tend to shift position as the test head is moved and rotated, and thus tend to offset the balance of the test head. If the test head were biased in its compliance range with springs, the springs would need to be extremely stiff to resist the large turning moment of the test head and the offsetting forces from the cables. However, we have recognized that stiff springs cause movement of the test head to become stiff, and thus non-compliant. Another solution is needed to satisfy the demands of gentle, compliant docking.